Voltage-tuneable magnetron



Nov. 24, 1964 w. A. GERLACH 3,158,780

VOLTAGE-TUNEABLE MAGNETRON Filed Dec. 21, 1961 3 Sheets-Sheet 1 INVENT OR Haas? MM. 659mm W, Q-JQFWF as 41 H 11 x 2 LM m 9, a M H m. w m an 1 m m w J Nov. 24, 1964 Filed Dec. 21, 1961 H. W. A. GERLACH VOLTAGE-TUNEABLE MAGNETRON :5 Sheets-Sheet 2 ROTATI N6 ELECTRON STREAM H0257 I404. GEPLACH INVENI'OR Nov. 24, 1964 H. w. A. GERLACH 3,153,780

VOLTAGE-TUNEABLE MAGNETRON Filed Dec. 21, 1961 3 Sheets-Sheet 3 H0257 14M. GEQL/ICH INVENTOR United States Patent sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty there- This invention relates to microwave signal sources and especially to improvements in magnetrons.

One well known type of voltage tuneable magnetron has an anode formed by a single pair of interdigital finger elements representing, together with the sole, its R-F interaction space. An annular beam of electrons is injected from one end into the interaction space. These electrons are rotated by means of the combination of a radial electric field and an applied magnetic field. However; because of the injection the motion of the electrons is not entirely azimuthal, a component of electron velocity v being directed along the longitudinal aXis of the anode fingers. in prior magnetrons of this type, even the lowest am'al or longitudinal electron velocity v attainable is large enough to seriously restrict the time of interaction or the beam with the 31-? signal wave. As a result, the amount of output power available is unduly limited. Attempts to increase anode eficiency and output power by lengthening the anode fingers meet with little success due to moding difiiculties which such measures create. The length of the anode fingers is also limited by other design considerations determining the desired operating frequency.

An object of this invention is to provide a new and improved magnetron.

Another object is to increase the power output of voltage-tunable magnetrons.

A further object is to provide a magnetron oscillator having a uniform level of output power over a wide frequency range.

Still another object of the present invention is to reduce end radiation losses of high power magnetrons.

An additional object is to provide a voltage-tuneable magnetron having improved efficiency as compared with known single interdigital anode magnetrons.

A further object is to'improve the coupling betv es the R-F vanes in the interaction region of a magnetron and its coaxial output line.

The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

FIG. 1 is a vertical sectional view of a magnetron constructed in accordance with the present invention shown mounted within its external microwave cavity. This cross-section is taken along centrfl axis XX of FIGS. 2, 4, and 6.

FIG. 2 is a plan view of the microwave cavity casing which serves to support the magnetron tube, with its top plate and the magnetron tube removed.

FIG. 3 is a sectional view of the casing taken along line 33 in FIG. 2.

FIG. 4 is an isometric view of the microwave cavity casing with its top plate in place.

FIG. 5 is a diagram in cross-section of the interdigital anode fingers.

FIG. 6 is an isometric view or" one form or" the anode fingers according to this invention.

3,158,780 Patented Nov. 24, 1964 "ice FIG. 7 is an alternate embodiment of the anode finger elements.

Referring now in detail to the embodiment of the invention illustrated in FIG. 1, the numeral 19 generally designates a voltage-tuneable magnetron. The various electrodes and other components of the magnetron tube are mounted within a microwave cavity formed by an external casing. The casing includes a base member 11, a center conductor plate 21, and a top plate 28. Base 11 comprises a bottom plate 12, two side walls 13 and 14 (FIGS. 2 and 3), and one end wall 15, whose inner surfaces form boundaries of the aforesaid microwave cavity. End wall 15 has a rectangular groove 16 located in its upper edge, as shown in FIGS. 3 and 4, which receives and supports one end of center plate 21. Bottom plate 12 contains a pair of concentric holes 17 and 18 which provide a lar e circular opening with a projecting lip 19, as best seen in F168. 1 and 3. Center conductor plate 21 is a fiat plate of rectangular shape with an L-shaped cross section formed by an upstandin wall at one end, its left end in FIG. 1. Center plate 21 contains an aperture 25 with a projecting lip 26 of slightly larger inside diameter than that of lip 19. A cylindrical conductor 22 slotted at one end and affixed to plate 21, as shown in FIGS. 1 and 2, serves as an output connector for the magnetron. Plate 21 rests in groove 16 at its left end and is spaced from the base 12 at its right end by insulator blocks 23 and 24 (see FIG. 2). The top plate 2% is supported on walls 13, 14, and 15 Where it completes the formation of the microwave cavity. A large aperture 29 in top plate 28 is provided with threads 39 and has a projecting lip 31 at its bottom edge. A ring nut 32 in the form of a fiat disk has external threads so that it meshes with threads 30 within aperture 29, for a purpose appearing later.

The magnetron tube structure is centrally mounted within apertures 13, 25' and 2? in order to place its R-F circuit inside the microwave cavity. Considering now the internal construction of the magnetron tube, a cylindrical sole 35 maintained at zero R-F potential is held in position by means described subsequently, along the central axis Y-Y' of the microwave cavity. The anode is an interdigital R-F circuit having two axially aligned rings of interleaving fingers, or vanes. This interdigital struc ture effectively has three terminals, the three conducting discs 37, 39 and 1 serving as the aforementioned terminals. There are three sets of anode fingers 36, 38 and 4t) (PEG. 6), each finger extending, respectively, from the inner rim of one of the discs 37, 39 and 41. The fingers, or vanes, are integral extensions of their respective discs, formed at right angles with the disks 37, 39 and 41. Center terminal disc 39 carries the first set of fingers 38 attached thereto at the mid-points of the fingers, as clearly appears in FIGS. 1 and 6. Top disc 37 supports a like number of fingers 36 which project downward in an interleaved fashion between fingers 38. Fingers 46 on the bottom disc 41 extend upward in axial alignment with vanes 3%. FIG. 5 illustrates in cross-section the alternation of fingers 38 with the pairs of fingers 36 and 4%, proceeding around the inner edge of the terminal discs. The 38-? structure as, 35, is suitably spaced radially to define an interaction region of proper size for the magnetron. Fingers 3% together with the upper halves of the fingers 38 form one interdigital line within the top section of the microwave cavity, while fingers 4% with the lower halves of fingers 38 act as another such line, in the cavity section below center plate 21. These two anode lines are effectively positioned back-to-baclr within cavity sections which are so tightly coupled together that only a single resonance appears. The result of this novel arrangement is that the axial length of the interaction is substantially increased without lengthening the anode fingers.

A pair of ceramic insulating rings 45 and 45 (FIG. 1) serve as spacing members to hold discs 37, 39 and 41 in their proper positions, as shown. Disc 41 fits inside hole 18 in base 11, seating upon lip 19, while lip 26 similarly supports terminal disc 39. An annular connector 45 of U-shaped'cross-section rests within aperture .29, mounted upon lip 31 with its inner rim 49 overlapping disc 37. Connector 48 electricmly couples terminal 37 to the microwave cavity casing at 28. The disc 41 being of reduced outside diameter, the tube unit can be inserted from above through aperture 29 into the cavity until discs 39 and 41 abut, respectively, lips 26 and 1?. Then connector ring 48 is fitted in place upon lips 31 and disc .37, and aforementioned ring nut 32 is threaded into threads 35? to lock ring 45 and the magnetron'tube against axial displacement. The magnetron tube itself further contains a control anode 52. and a cathode 65. Control anode 52 is a disc having an internal depending conical portion 53. Its position relative to sole 35 and the R-F anode is established by a spacing ring 51 similar to insulators 4-5 and 46. Another ceramic ring 54 resting upon control anode 52 carries a circular metal plate 55 with a small central hole having two counterbores to form a pair of lands therein. A thin Walled metal tube 56 has an upper rim 57 fitting the inner land of plate 55 to mount the tube 55 inside the plate hole. An insulating washer 65 seated upon the outer land of plate 55 and on rim 57 supports a member 61 centrally along axis Y-Y'. Member 61 is a cylindrical rod witha reduced end portion 62 and a fiat circular head 63 overlapping washer 50. A cathode filament 65 is helically wound in the form of an inverted conical frustrum, as illustrated in FIG. 1. At the enlarged upper end, cathode 65 fits tightly over tube 56, the other end of the filament extending inwardly to coil around end 62 of rod 61. The cathode filament 65 is thus disposed symmetrically respecting axis Y-Y' and aligned closely above sole 35,.

with control anode 52 situated between the cathode and the interaction region. Cathode 55, control anode 52, sole 35, the R-F anode, the spacer rings and the various other internal mounting elements form a unitary structure, which is referred to herein as the magnetron tube.

The opposite end of the magnetron tube comprising the support for sole 35 will now be described, The lower part of sole 35 is of smaller diameter, forming an end rod 35. A retaining washer 7% of conductive material is force fitted upon the lower extremity of rod 35', where it serves to support another insulating washer 71 and a metallic disk 72 on rod 35. Disk 72 has a center hole counterbored underneath to receive the upper surface of washer 71. An insulating spacer ring 47 mounted on an upper shoulder of disk 72 abuts terminal disc 41 of the R-F anode. Thus, by axially adjusting end 35 in retaining washer 70, sole 35 is positioned correctly within the magnetron tube and firmly held along axis Y-Y by members 70, 71, 72 and 47.

ing an external circular shoulder 78. a

An insulator 79, an annular member with an outer flange giving it an L-shaped cross section, is placed within hole 17 in plate 12. When pole piece 74 is inserted Within member 79 until shoulder 78 contacts the latter,

the lip 76 firmly engages disk 72 to hold thepole piece in place. Washer 70 rests on the bottom of bore 75 with 35' extending into hole 77. An insulator 89 identical with member 79 surrounds pole piece 74 on the opposite side of the shoulder 78.

An annular connector 8 3 of conductive material is provided with a central aperture 84, a projecting lip 85 inside the aperture, and a flared side Wall 86 imparting at the main anode.

jection anode, such as control anode 52.

of the magnetron tube, once the connector 83 has been mounted onanode 52. A bore 91 in the smaller end of member 91'? has a rim 32 adapted to engage circular plate 55 of the tube unit. A strong magnetic field directed along axis Y-Y' is established in the gap between polepieces 9% and 74, which are composed of high reluctance magnetic material. Any well known type of magnetic field source may be used to apply the field to pole pieces 74 and 9%). An additional magnetic insert 94 serves to further direct and concentrate the magnetic field in the interaction region between sole 35 and the R-F anode elements 35, 3S, and as. Member 94 has the form of a thick ring with a depending ridge, the opposite side being afiixed to a, groove in plate 55.

The operation of the magnetron shown in FIGS. 1-6 will now be briefiy indicated. (Suitable external electrical connections (not shown) are brought out from plate 55 and rod 61 for coupling cathode filament 65 to a filament power supply.) The cold cathode or sole 35 and filament rod 61 are at zero potential. With a predetermined positive 110. voltage applied through connector' 83, the injection control anode, 52 surrounding emitting cathode 65 serves to focus emitted electrons into a beam which encircles sole 35 and travels toward the lower end disk 72. The electrons follow a helical path which increases in radius, expanding toward the R-F anode in the presence of themagnetic field. I

As is well known, in voltage tuneable magnetrons, it is necessary to provide an electron beam in which the current is independent of the DC. voltage variations The injected-beam tubes achieve this condition by controlling beamcurrent with an in- Even though the applied voltage of the R-F structures 35, 33, 49 is large, anode 52 shields the cathode from the electric D.C. field due to its position in proximity to the cathode 65. Thus, the potential of control anode 52 deter-mines the operating point of emitting cathode 65 of the temperature limited characteristic. Constant beam current is assured even with variations of the voltage of the interdigital structure, thus allowing voltage tuning of the operating frequency.

An important feature provided in accordance with this invention, is the particular conical configuration imparted to emitting cathode It has been discoveredthat this filament shape, combined with the focussing of control anode 52, 53 and magnetic insert 94 gives a nearly uniform current distribution along the cathode surface.

That is, with an optimum cathode angle of the filament 65, uniform cathode loading is obtained. That is not true in prior art cylindrically wound cathode filaments of constant radius, whereby the portion next to the anode structure is more heavily loaded and a non uniform cathode loading results.

As the electrons move radially outward from sole 35, they are retarted by the induced R-F fields in interdigital fingers 36, 33, 4t) and, on the average, transfer energy to the R-F fields to sustain them. Since both fingers 4t and 36 are electrically connected to the ground plane of the microwave cavity casing 11, and since these fingers are short in comparison with the operational R-F wavelength,

the top and bottom terminal discs 37 and 41 areat the same R-F potential. The center disc 39 is out of phase with the other discs. As a result, 1r-mode operation occurs in the magnetron, with alternate vanes 36, 38 and 4% being separated electrically by rr-radians. FIG. 5

illustrates the vertical alignment of the two sets of interlocked anode vanes or fingers, with the beam intermediate them and the sole 35. From FIG. 6 it can be seen that the two interdigital lines, one comprising terminal disc 39, fingers 3S, fingers 40 and disc 41 and the other formed by disc 37, fingers 36, fingers 38 and disc 39, are placed back-to-back so that they are operated in a push-pull manner. Their cavity sections, upper and lower portions of the microwave cavity defined by casing 11 and only partially separated by plate 21, are thereby coupled together so tightly or closely that these two R-F anode sections operate at a single frequency.

Thus, with this arrangement, the length of the interaction region is practically doubled, giving increased power output and efiiciency without the drawbacks inherent in lengthening individual fingers. The magnetron structure is geometrically symmetric about the center plane through plate 21; this causes the transition from the TE mode of the interaction region to the TEM mode in the coaxial output to be much smoother, i.e. less frequency sensitive. The coupling to the R-F structure is improved, resulting in a uniform power-frequency response curve, particularly desirable for voltage-tuneable magnetrons. A symmetrical output signal in correct phase is provided without the necessity of using transformers. In addition, radiation of leakage R-F power from the ends of the new magnetron device has been eliminated or severely attenuated, whereas conventional magnetrons waste a substantial part of the generated power through leakage losses.

An alternate embodiment of the R-F anode structure is illustrated in FIG. 7, wherein like reference numerals indicate the same parts as in FIGS. 1 and 6. The terminal discs 37, 39, and 41 are the same, as are fingers 38. However, it has discovered that the alternate fingers need not be separated into two sections. Thus solid fingers 42, intergrally connected at their ends to discs 37 and 41, replace the separate sets of fingers 36 and 4%, there now being two sets of fingers. However, efiectively two backto-back interdigital lines are still present, the first comprising the upper parts of fingers 33 and the upper halves of fingers 42, the other including the bottom halves of these fingers. Note that, in the prior embodiment, fingers 36 and 4% were at the same R-F potential, both being connected to the cavity casing.

While ceramic insulating spacers have been shown for electrically isolating and positioning various tube components, it is equally possible to rigidly mount the various electrodes, as by brazing or welding, to support members of the cavity and omit the spacer rings, such as 45, 46, 47, 51 and 54. In this event, the magnetron should be placed in an evacuated microwave cavity. In the embodiment of FIGS. 1 and 6, only the magnetron tube within the spacer rings need be evacuated.

The voltage-tuneable magnetron of the present invention thus affords a significant improvement in efiiciency, greater power output capability, reduced end radiation losses, and a smoother tuning response characteristic. The physical arrangement of the components features ease of assembly and dis-assembly, so that if a component is faulty it may be quickly and simply replaced. By merely lifting off pole piece 90 and connector 83, disengaging ring nut 30 and removing connector ring 43, a defective magnetron tube can be replaced as a unit in the microwave cavity. Also, the electron discharge device described herein may be employed either as an amplifier or as an oscillator.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

I claim as my invention:

1. An electron discharge device of the voltage tuneable magnetron type comprising in combination;

(a) cavity means,

(b) cold cathode means,

(0) electron beam injection means disposed at one end of said cold cathode means,

(d) means for applying a magnetic field within said cavity means,

(e) a cylindrical anode assembly surrounding said cathode means centrally disposed in said cavity means, said anode assembly including a series of axially extending anode fingers, said anode fingers connected to form two sets of said anode fingers, said sets of anode fingers interdigitated with respect to one another,

(f) an upper disc, at lower disc, and a central disc,

(g) a first set of anode fingers connected to said upper and said lower discs, and the second set of anode fingers connected to said central terminal disc,

(/2) broad band output means including TEM mode transmission means having an upper ground plane and a lower ground plane, and a centrally disposed conductor, said upper ground plane connected to and surrounding said upper disc, said. lower ground plane connected to and surrounding said lower disc, and said centrally disposed conductor connected to and surrounding said central disc.

2. A voltage tuneable magnetron as in claim 1 wherein said electron beam injection means comprises a cone shaped, helically wound filament means, the apex of said filament means being adjacent said cold cathode means.

3. A magnetron as in claim 1 wherein said first set of anode fingers includes a continuous bar extending from said upper disc to said lower disc.

4. An electron discharge device of the voltage tuneable magnetron type comprising in combination;

(a) cavity means,

(b) cold cathode means,

(0) electron beam injection means disposed at one end of said cold cathode means,

((1) means for applying a magnetic field within said cavity means,

(e) a cylindrical anode assembly surrounding said cathode means centrally disposed in said cavity means, said anode assembly including a series of axially extending anode fingers, said anode fingers connected to form two sets of said anode fingers, said sets of anode fingers interdigitated with respect to one another,

(1) an upper disc, a lower disc, and a central disc,

(g) a first set of said anode fingers connected to said upper and said lower discs, and the second set of anode fingers connected to said central-terminal disc,

(11) broad band output means including TEM mode transmission means having an upper ground plane and a lower ground plane, and a centrally disposed conductor, said upper ground plane connected to and surrounding said upper disc, said lower ground plane connected to and surrounding said lower disc, and said centrally disposed conductor connected to and surrounding said central disc, and

(i) said first set of anode fingers comprised of one set of fingers extending downward from said upper disc and another set extending upward from said lower disc in axial alignment with said one set of fingers.

References Qited in the file of this patent UNITED STATES PATENTS 2,463,416 Nordsieck Mar. 1, 1949 2,810,096 Peters et al. Oct. 15, 1957 3,013,180 Peters Dec. 12, 1961 3,084,280 McLaughlin Apr. 2, 1963, 

4. AN ELECTRON DISCHARGE DEVICE OF THE VOLTAGE TUNEABLE MAGNETRON TYPE COMPRISING IN COMBINATION; (A) CAVITY MEANS, (B) COLD CATHODE MEANS, (C) ELECTRON BEAM INJECTION MEANS DISPOSED AT ONE END OF SAID COLD MEANS, (D) MEANS FOR APPLYING A MAGNETIC FIELD WITHIN SAID CAVITY MEANS, (E) A CYLINDRICAL ANODE ASSEMBLY SURROUNDING SAID CATHODE MEANS CENTRALLY DISPOSED IN SAID CAVITY MEANS, SAID ANODE ASSEMBLY INCLUDING A SERIES OF AXIALLY EXTENDING ANODE FINGERS, SAID ANODE FINGERS CONNECTED TO FORM TWO SETS OF SAID ANODE FINGERS, SAID SETS OF ANODE FINGERS INTERDIGITATED WITH RESPECT TO ONE ANOTHER, (F) AN UPPER DISC, A LOWER DISC, AND A CENTRAL DISC, (G) A FIRST SET OF SAID ANODE FINGERS CONNECTED TO SAID UPPER AND SAID LOWER DISCS, AND THE SECOND SET OF ANODE FINGERS CONNECTED TO SAID TERMINAL DISC, 